Peppermint: Effect of time of harvest on yield and quality of oil

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Abstract
In a field experiment with peppermint (Mentha piperita L.), 13 weekly harvests were taken from 3 January to 28 March 1979 and measurements of dry matter yield, stem length, leaf: stem ratio were made, together with yield and quality of peppermint oil. The best yield of high quality oil was obtained in late January–early February when the inflorescence on the main stem was in 10–20% bloom. Oil yield peaked at 114 kg ha-1 on 31 January. At this time the composition of extracted oil was: menthol 43.1, menthone 28, menthyl acetate 6.6, cineole 4.1, and menthofuran 3.8%. After this date oil yield declined steadily because of leaf senescence, while the percentage of menthol, menthyl acetate, and menthofuran increased, and menthone decreased. It is concluded that a combination of plant maturity and menthol level should be used to determine time of harvest.